how to make a guy regret rejecting you?

[Joe]

This probably isn't what you want to hear. Oh well.
Trying to make him regret rejecting you is bad for two reasons.
First and foremost, the whole idea of it shows that you are wrapped up in what this guy thinks of you. I know you might have feelings for him but it sounds like he's just into someone else right now, and I hate to say it but when that happens there isn't much you can do. You can either keep him friend zoned and try to preserve what you have, or you can distance yourself from him (i.e. stop talking to him as much, or altogether) and wait it out while you try things out with other guys.
If you only like him as a friend then I think you should let him pursue his romantic interest. I know I wouldn't have wanted to go to prom with a girl I was "just friends" with in high school. Prom is generally thought of as a romantic event.
The second reason I think trying to make him regret rejecting you is a bad idea is that it will almost definitely backfire. When someone does something like this it makes them look selfish and immature, and he'll probably just label you as another crazy girl instead of actually regretting anything (this is the typical high school male mentality). Just try to relax and have a good time at prom. I wish I hadn't got caught up in so much stupid drama in high school. You'll never get these years back so try to take it in your stride and stay positive